Integration type input circuit and method of testing it

ABSTRACT

An input interface circuit is provided with an input transistor for receiving a digital input signal, a circuit for generating a reference value, and an integrating capacitor connected in series to a pair of current conducting electrodes of the input transistor for integrating the input signal, a logical level of the input signal being discriminated by comparing an integration of the input signal with the reference value. For a testing function mode, a test transistor is connected to a junction between the pair of current conducting electrodes of the input transistor and the integrating capacitor so that a current driving capability is determined. Also, a discharge path circuit for controllably discharging the integrating capacitor is connected to the junction between the input transistor and the integrating capacitor. A method of testing an input interface circuit provided with an input transistor for receiving a digital input signal, a circuit for generating a reference value, an integrating capacitor connected in series to a pair of current conducting electrodes of the input transistor for integrating the input signal, a test transistor connected to a junction between the pair of current conducting electrodes of the input transistor and the integrating capacitor, and a discharge path circuit for controllably discharging the integrating capacitor is connected to the junction between the input transistor and the integrating capacitor includes the steps of operating one of the input and test transistors so that fixed current flows through a pair of current conducting electrodes of the one transistor, operating the other transistor by applying a sequentially varying signal to the other transistor so that current flowing through a pair of current conducting electrodes of the other transistor is sequentially varied, comparing an output of the integrating capacitor with the reference value to determine a logical level of the input signal, and finding a value of the sequentially varying signal when the determined logical level is changed from one state to another.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to U.S. Serial No. 08/ filed on Oct. 10, 1997, claiming the priority based on Japanese Patent Application No. 8-289131 filed on Oct. 13, 1996 and assigned to the assignee of the present application, the whole of the disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a digital signal input circuit and a method of testing it, and more particularly to an input interface circuit to which a high-speed and small-amplitude digital signal is applied.

[0003] For example, a personal computer includes functional IC blocks such as a CPU, a memory controller and a memory. The memory controller controls an access from the CPU or the other device to the memory. The transfer of data between the CPU and the memory controller and the transfer of data between the memory controller and the memory are performed through buses, respectively.

[0004] Data received by each of these or similar IC blocks from the other IC block is in the form of a high-speed and small-amplitude digital signal. Since the amplitude of the signal is too small, the data cannot be handled by a circuit in the IC block as it is. Especially, if an inputted digital signal includes noise superimposed thereon, the judgement of a logical level thereof will be difficult. Accordingly, it is general that an input portion of the IC block such as a memory controller or a memory is provided with an input interface circuit which performs discrimination and waveform shaping of an inputted digital signal.

[0005] Many conventional input interface circuits includes a combination of a current mirror type differential amplifier and a latch circuit. In this interface circuit, a logical level of an input signal having a small peak-to-peak voltage of, for example, about 0.8 V is discriminated through the comparison thereof with a reference signal by use of the current mirror type differential amplifier, the discriminated input signal is amplified up to a voltage of, for example, about 2.5 V to 3.0 V which can be handled by a digital circuit, and the amplified output signal is held by the latch circuit which is a digital circuit. The test of the operation of such a differential input type input interface circuit includes a test through which the judgement is made of whether or not a set-up operation from a point of time of stabilization of an output of the differential amplifier to a point of time when it can be held by the latch circuit falls within a specified set-up time.

[0006] The judgement is made on the basis of two criteria. One is concerned with the timing of the set-up operation, that is, whether or not a transistor in the input interface circuit has a sufficient driving capability for discrimination of a logical level in a prescribed time. The other is concerned with whether or not the sampling for the latch circuit of the input interface circuit is performed in the prescribed time.

[0007] For such a test of the operation of the input interface circuit, there has hitherto been used a memory tester which generates an analog signal having a linearly varying rise pattern and applies this analog signal to the input interface circuit in a manner such that the applied analog signal is sequentially shifted by a short time to measure a time until a normal operation is no longer available.

[0008] However, the conventional tester provided with the above-mentioned function of generating an analog signal capable of short-time shifts has a complicated circuit structure and is therefore high in cost. Accordingly, an input interface circuit completed through a test using such a costly tester becomes expensive.

[0009] Further, JP-A-8-63268 (laid open on Mar. 8, 1996) has disclosed an input/output interface circuit provided with an integrating circuit. However, no reference is made to the test of that circuit.

SUMMARY OF THE INVENTION

[0010] One object of the present invention is to provide an input interface circuit having a function for facilitating a test.

[0011] The input interface circuit according to the present invention may be used with the above-mentioned digital IC block.

[0012] Another object of the present invention is to provide a method of testing an input interface circuit.

[0013] According to one aspect of the present invention, an input interface circuit is provided with an input transistor for receiving a digital input signal, a circuit for generating a reference value, and an integrating capacitor connected in series to a pair of current conducting electrodes of the input transistor for integrating the input signal, a logical level of the input signal being discriminated by comparing an integration of the input signal with the reference value. For a testing function mode, a test transistor is connected to a junction between the pair of current conducting electrodes of the input transistor and the integrating capacitor so that a current driving capability is determined. Also, a discharge path circuit for controllably discharging the integrating capacitor is connected to the junction between the input transistor and the integrating capacitor.

[0014] According to another aspect of the present invention, a method of testing an input interface circuit provided with an input transistor for receiving a digital input signal, a circuit for generating a reference value, an integrating capacitor connected in series to a pair of current conducting electrodes of the input transistor for integrating the input signal, a test transistor (N69) connected to a junction between the pair of current conducting electrodes of the input transistor and the integrating capacitor, and a discharge path circuit for controllably discharging the integrating capacitor is connected to the junction between the input transistor and the integrating capacitor, includes the steps of:

[0015] operating one of the input and test transistors so that fixed current flows through a pair of current conducting electrodes of the one transistor;

[0016] operating the other transistor by applying a sequentially varying signal to the other transistor so that current flowing through a pair of current conducting electrodes of the other transistor is sequentially varied;

[0017] comparing an output of the integrating capacitor with the reference value to determine a logical level of the input signal, and;

[0018] finding a value of the sequentially varying signal when the determined logical level is changed from one state to another.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a block diagram of an input interface circuit according to an embodiment of the present invention.

[0020]FIG. 2 is a block diagram of an input interface circuit according to another embodiment of the present invention.

[0021]FIG. 3 is a block diagram of an input interface circuit according to a further embodiment of the present invention.

[0022]FIG. 4 is a diagram for explaining the operation of the embodiment shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] In FIG. 1 showing a block diagram of an input interface circuit according to an embodiment of the present invention, an input circuit section 1 includes first and second transistors P53 and P54 which are, for example, P-type MOS transistors. Each of the transistors P53 and P54 has a pair of current conducting electrodes (or source/drain electrodes) and a control electrode (or gate electrode). The transistor P53 receives a digital input signal and the transistor P54 receives a reference signal. The sources of the transistors P53 and P54 are connected to a precharge transistor P52 (for example, P type) in a precharge circuit section 2. The drains of the transistors P53 and P54 are respectively connected through an input signal line L1 and a reference signal line L2 to input terminals of a differential amplifier included in a sense amplifier 3. The signal lines L1 and L2 are connected to the gates of transistors N56 and N57 (for example, N-type MOS transistors) in the precharge circuit section 2 and are discharged in accordance with a control signal supplied to the gates of the transistors N56 and N57 from a control terminal 21. An N-type transistor N58 serves to hold the signal lines L1 and L2 at the same potential in a state in which the signal lines L1 and L2 are precharged (or reset), that is, a state in which capacitors PAR62 and PAR63 mentioned later on are discharged. The transistors N56, N57 and N58 form a discharge path circuit for the capacitors PAR62 and PAR63. Accordingly, this discharge path circuit is connected to a junction between the transistor P53 and the integrating capacitor PAR62 and to a junction between the transistor P54 and the integrating capacitor PAR63.

[0024] The signal lines L1 and L2 are also connected to a test circuit section 4. The test circuit section 4 includes MOS transistors N69 and N70 of a conduction type (in the shown example, N type) opposite to that of the first and second transistors P53 and P54 in the input circuit section 1. The sources of the transistors N69 and N70 are connected to the signal lines L1 and L2, respectively. Each of the transistors N69 and N70 has a pair of current conducting electrodes (or source/drain electrodes) and a control electrode (or gate electrode).

[0025] In order to facilitate the understanding, it is shown in FIG. 1 that the capacitors PAR62 and PAR63 are connected to the signal lines L1 and L2, respectively. For these capacitors, use may be made of stray capacitances provided by paths extending from the first and second transistors P53 and P54 of the input circuit section 1 to the sense amplifier (or differential amplifier) 3. A test pad 41 is connected to the gate of the N-type MOS transistor N69 and a test pad 42 is connected to the gate of the N-type MOS transistor N70.

[0026] In the present embodiment, the integrating capacitor PAR62 is a stray capacitance formed between a common potential source and a junction between the transistor P53 and the transistor N69, and the integrating capacitor PAR63 is a stray capacitance formed between the common potential source and a junction between the transistor P54 and the transistor N70.

[0027] In a usual input signal discriminating operation of the circuit shown in FIG. 1, the precharging (or resetting) of the signal lines L1 and L2 by the precharge circuit section 2 is first performed. Namely, the capacitors PAR62 and PAR63 are discharged through the discharge path circuit (N56 to N57) of the precharge circuit section 2. Next, with the signal lines L1 and L2 being precharged by the turn-on of the precharge transistor P52, a supply voltage Vdd is supplied to the sources of the transistors P53 and P54. When the transistors P53 and P54 are turned on in response to an input digital signal and a reference signal supplied to input terminals INPUT and Vref, the capacitors PAR62 and PAR63 are charged in accordance with the amplitudes of the input digital signal and the reference signal. This charging operation corresponds to an integrating operation. Voltages developed by the charged capacitors PAR62 and PAR63 are applied to the sense amplifier 3 which in turn amplifies a difference between both the voltages as the result of comparison thereof. In accordance with an external clock signal, data is latched by a latch circuit included in the sense amplifier 3. As a result, an output of a logical level corresponding to a difference in potential between the reference signal line L2 and the input signal line L1 is taken out of the sense amplifier 3.

[0028] The control of the charging/discharging (or integration time) of the capacitors PAR62 and PAR63 and the turn-on/off of the discharge circuit is performed in accordance with a control clock signal supplied to the control terminal 21.

[0029] In the usual input signal discriminating operation mentioned above, test pads 41 to 44 in the test circuit section 4 are placed in an open state by switches SW1 to SW4. Accordingly, the transistors N69 and N70 have no influence on the capacitors PAR62 and PAR63.

[0030] Next, description will be made of a testing function mode of the input interface circuit shown in FIG. 1.

[0031] (1) Test of First Transistor P53 in Input Circuit Section 1 for Receiving Input Signal

[0032] Voltages similar to the two input voltages in the usual input signal discriminating operation mode, for example, voltages of 1.4 V±0.4 V and 1.4 V are applied to the input terminals INPUT and Vref, respectively. A signal having a logical level of HIGH is supplied to the test pad 41 by the switch SW1 and a voltage source E1 so that the transistor N69 is turned on. Signals of a logical level of LOW are respectively supplied to the test pad 42 by the switch SW2 and the voltage source E1 and to the test pad 44 by the switch SW4 and a voltage source E2 so that the transistor N70 is brought into a turned-off condition.

[0033] A test signal is applied to the test pad 43 by the switch SW3 and the voltage source E2. The voltage applied to the test pad 43 is sequentially varied to find whether an output of the sense amplifier 3 is a logic “1” or a logic “0”. Namely, the voltage applied to the test pad 43 is sequentially increased or decreased so that the value of a voltage applied to the test pad 43 at the time of change of the output of the sense amplifier 3 from the logic “1” to the logic “0” or from the logic “0” to the logic “1” is read by, for example, a voltmeter. This voltage value can be regarded as an index which represents a current driving capability of the transistor P53. If the current driving capability represented by this index is not out of a tolerance of a design value, it is proved that the operation of the first transistor P53 in the input circuit section is ensured.

[0034] Namely, since the transistors P53 and N69 are transistors having different conduction types, the potential of a junction between both the transistors represents a difference in driving capability between both the transistors. Accordingly, the current driving capability is determined from the value of a particular value of the applied test voltage when the output of the sense amplifier is inverted in the case where one of the two transistors is held in a fixed-current operation state while varying an operating current of the other transistor, the particular value of the applied voltage determining an operating current of the other transistor at the time of inversion of the sense amplifier output.

[0035] Accordingly, the current driving capability of the transistor P53 can also be determined by applying a fixed voltage to the test pad 43 while varying the voltage of a signal applied to the transistor P53 (i.e., the input terminal INPUT) so that the value of a voltage applied to the test pad 43 at the time of change of the output of the sense amplifier 3 from the logic “1” to the logic “0” or from the logic “0” to the logic “1” is read by, for example, a voltmeter, as mentioned above.

[0036] (2) Test of Second Transistor P54 in Input Circuit Section 1 for Receiving Reference Signal

[0037] Similarly to the case of the test (1), two input voltages in the usual input signal discriminating operation mode are applied to the input terminals INPUT and Vref, respectively. A signal having a logical level of HIGH is supplied to the test pad 42 by the switch SW2 and the voltage source E1 so that the transistor N70 is turned on. Signals of a logical level of LOW are respectively supplied to the test pad 41 by the switch SW1 and the voltage source E1 and to the test pad 43 by the switch SW3 and the voltage source E2 so that the transistor N69 is brought into a turned-off condition.

[0038] A test signal is applied to the test pad 44 by the switch SW3 and the voltage source E2. The voltage applied to the test pad 43 is sequentially varied to find whether an output of the sense amplifier 3 is a logic “1” or a logic “0”. Namely, the voltage applied to the test pad 44 is sequentially increased or decreased so that the value of a voltage applied to the test pad 44 at the time of change of the output of the sense amplifier 3 from the logic “1” to the logic “0” or from the logic “0” to the logic “1” is read by, for example, a voltmeter. This voltage value can be regarded as an index which represents a current driving capability of the transistor P54. If the current driving capability represented by this index is not out of a tolerance of a design value, it is proved that the operation of the second transistor P54 in the input circuit section is ensured.

[0039] Regarding the test of the transistor P54 too, the current driving capability of the transistor P54 can also be determined, in a manner similar to that in the test of the transistor P53, by applying a fixed voltage to the test pad 44 while varying the voltage of a signal applied to the transistor P54 (i.e., the input terminal Vref) so that the value of a voltage applied to the test pad 43 at the time of change of the output of the sense amplifier 3 from the logic “1” to the logic “0” or from the logic “0” to the logic “1” is read by, for example, a voltmeter, as mentioned above.

[0040]FIG. 2 shows a modification of the embodiment shown in FIG. 1.

[0041] Namely, in place of the second transistor P53 of the input circuit section 1 and the integrating capacitor PAR63, an amplifier AMP including a low-pass filter is provided between the input terminal Vref and the signal line L2.

[0042] It is not necessarily required that a reference signal used for discrimination (or comparison) of a logical level of an input signal should be integrated. As shown in FIG. 2, an output of the amplifier for amplifying the reference signal to a value to be referenced to is applied to one input of the sense amplifier 3, while the input signal is integrated, as in the embodiment shown in FIG. 1, in accordance with a control signal supplied to the control signal 21 so that the integration is applied to another input of the sense amplifier 3 through the signal line L1.

[0043] The test of the input circuit section 1 of the input interface circuit shown in FIG. 2 may be performed by conducting the test of only the input transistor P53 in a manner similar to that in the test of the first transistor of the input interface circuit shown in FIG. 1.

[0044] Next, description will be made of the control clock signal supplied to the control terminal 21.

[0045]FIG. 3 shows a semiconductor integrated circuit device which includes a plurality of circuits 10 similar to the input interface circuit shown in FIG. 1 or 2 and a control signal generating circuit 12.

[0046] The control signal generating circuit has a PLL circuit and receives an external clock signal at a clock terminal 11 to generate a control clock signal which is synchronous in phase with the external clock signal. This control clock signal is outputted to a test pad 14 through a MOS transistor 13 of, for example, N type and is also supplied to control terminals of the input interface circuits 10 so that it is used for the control of the charging/discharging of an integrating capacitor and the turn-on/off of a discharge path circuit in each of the circuits 10. A reference signal pad 15 corresponds to the Vref terminal in FIG. 1 and an input signal pad 16 corresponds to the INPUT terminal in FIG. 1. Also, an output test pad 17 is a terminal from which a signal corresponding to the output of the sense amplifier 3 in FIG. 1 is generated.

[0047] As mentioned above, a control signal used for the control of the charging/discharging of capacitors PAR62 and PAR63 and the turn-on/off of a discharge circuit in each input interface circuit is generated from the external clock signal. Since a digital input signal to each input interface circuit is inputted with a relation timed with the external clock signal, it is preferable that the control clock signal is as synchronous in phase with the external clock signal as possible. If the phases of the control and external clock signals extremely deviate from each other, the digital input signal discriminating function of the input interface circuit will be deteriorated.

[0048] Accordingly, in testing the input interface circuit, the control clock signal is extracted from the test pad 14 and is phase-compared by a monitor 30 with the external clock signal supplied to the clock input terminal 11 to verify a phasic relation therebetween.

[0049] For example, as shown in FIG. 4, the use of the control clock signal in a testing function mode is enabled in the case where the phase deviation of the control clock signal from the external clock signal is nearly zero as shown by a) of FIG. 4, in the case where it is −5% as shown by c) and in the case where it is +1% as shown by d). On the other hand, in the case where the phase deviation is +50% as shown by b), the use of the control clock signal in a testing function mode is disabled. The permissible limit for the control clock signal may be set properly in accordance with the design specification of the input interface circuit. For example, whether or not an integration voltages obtained using the control clock signal in regard to the first and second transistors P53 and P54 in the input circuit section 1 of the input interface circuit having a certain driving capability are sufficiently large, may be taken as a judgement criterion for the permissible limit. 

1. An input interface circuit having a testing function mode, an input signal being applied to said input interface circuit timed with an external clock signal, the input interface circuit comprising: a first transistor of a first conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the first transistor is conductive, wherein an input signal to the input interface circuit is to be applied to said control electrode of the first transistor; a second transistor of said first conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the second transistor is conductive, wherein a reference signal is to be applied to said control electrode of the second transistor; a first integrating capacitor connected in series to said pair of said current conducting electrodes of said first transistor, said input signal being integrated by said first integrating capacitor; a second integrating capacitor connected in series to said pair of said current conducting electrodes of said second transistor, said reference signal being integrated by said second integrating capacitor; a third transistor of a second conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the third transistor is conductive, one of said pair of current conducting electrodes being connected to a junction between said first transistor and said first integrating capacitor, wherein a first test signal is to be applied to said control electrode of the third transistor and a second test signal is to be applied to the other of said said pair of current conducting electrodes of the third transistor in a testing function mode, said junction between said first transistor and said first integrating capacitor serving to provide an output corresponding to an integration of said input signal to the input interface circuit; a fourth transistor of the second conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the fourth transistor is conductive, one of said pair of current conducting electrodes being connected to a junction between said second transistor and said second integrating capacitor, wherein a third test signal is to be applied to said control electrode of the fourth transistor and a fourth test signal is to be applied to the other of said said pair of current conducting electrodes of the third transistor in the testing function mode, said junction between said second transistor and said second integrating capacitor serving to provide an output corresponding to an integration of said reference signal; and a discharge path circuit connected to said junction between between said first transistor and said first integrating capacitor and to said junction between said second transistor and said second integrating capacitor, said first and second integrating capacitors being controllably discharged through said discharge path circuit.
 2. An input interface circuit according to claim 1, further comprising a control signal generating circuit which is responsive to said external clock signal to provide a control signal to be applied to said discharge path circuit for defining an integration time for said first and second integrating capacitors.
 3. An input interface circuit according to claim 2, wherein said control signal generating circuit has an external output end to provide a portion of said control signal for comparison with a portion of said external clock signal for verification of a phasic relation therebetween.
 4. An input interface circuit according to claim 1, wherein said first integrating capacitor is a first stray capacitance formed between a common potential source and the junction between said first transistor and said third transistor, while said second integrating capacitor is a second stray capacitance formed between the common potential source and the junction between said second transistor and said fourth transistor.
 5. An input interface circuit according to claim 1, further comprising a comparator connected to receive said outputs corresponding to integrations of said input and reference signals.
 6. An input interface circuit having a testing function mode, an input signal being applied to said input interface circuit timed with an external clock signal, the input interface circuit comprising: an input transistor of a first conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the input transistor is conductive, wherein an input signal to the input interface circuit is to be applied to said control electrode of the input transistor; an amplifier, wherein a reference signal is to be applied to said amplifier; an integrating capacitor connected in series to said pair of said current conducting electrodes of said first transistor, said input signal being integrated by said first integrating capacitor; a test transistor of a second conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the third transistor is conductive, one of said pair of current conducting electrodes is connected to a junction between said input transistor and said integrating capacitor, wherein an first test signal is to be applied to said control electrode of the test transistor and a second test signal is to be applied to the other of said said pair of current conducting electrodes of the test transistor in a testing function mode, said junction between said input transistor and said integrating capacitor serving to provide an output corresponding to an integration of said input signal to the input interface circuit; and a discharge path circuit connected to said junction between between said input transistor and said integrating capacitor, said integrating capacitor being controllably discharged through said discharge path circuit.
 7. An input interface circuit according to claim 6, further comprising a comparator connected to receive said output corresponding to an integration of said input signal and an output of said amplifier.
 8. A semiconductor integrated circuit device having a testing function mode, the device comprising: a plurality of input interface circuits having, an input signal being applied to each of said input interface circuit timed with an external clock signal, each of said input interface circuits including a first transistor of a first conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the first transistor is conductive, wherein an input signal to the input interface circuit is to be applied to said control electrode of the first transistor, a second transistor of said first conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the second transistor is conductive, wherein a reference signal is to be applied to said control electrode of the second transistor; a first integrating capacitor connected in series to said pair of said current conducting electrodes of said first transistor, said input signal being integrated by said first integrating capacitor, a second integrating capacitor connected in series to said pair of said current conducting electrodes of said second transistor, said reference signal being integrated by said second integrating capacitor, a third transistor of a second conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the third transistor is conductive, one of said pair of current conducting electrodes being connected to a junction between said first transistor and said first integrating capacitor, wherein a first test signal is to be applied to said control electrode of the third transistor and a second test signal is to be applied to the other of said said pair of current conducting electrodes of the third transistor in a testing function mode, said junction between said first transistor and said first integrating capacitor serving to provide an output corresponding to an integration of said input signal to the input interface circuit, a fourth transistor of the second conduction type having a control electrode and a pair of current conducting electrodes through which electric current flows when the fourth transistor is conductive, one of said pair of current conducting electrodes being connected to a junction between said second transistor and said second integrating capacitor, wherein a third test signal is to be applied to said control electrode of the fourth transistor and a fourth test signal is to be applied to the other of said said pair of current conducting electrodes of the third transistor in the testing function mode, said junction between said second transistor and said second integrating capacitor serving to provide an output corresponding to an integration of said reference signal, and a discharge path circuit connected to said junction between said first transistor and said first integrating capacitor and to said junction between said second transistor and said second integrating capacitor, said first and second integrating capacitors being controllably discharged through said discharge path circuit; and a control signal generating circuit which is responsive to said external clock signal to provide a control signal to be applied to the discharge path circuit of each of said input interface circuits for defining an integration time for said first and second integrating capacitors in each input interface circuit, said control signal generating circuit having an external output end to provide a portion of said control signal for comparison with a portion of said external clock signal for verification of a phasic relation therebetween.
 9. A semiconductor integrated circuit device according to claim 8, wherein each of said input interface circuit further includes a comparator connected to receive said outputs corresponding to integrations of said input and reference signals.
 10. An input interface circuit for discriminating a logical level of a digital signal inputted timed with an external clock signal by comparing said inputted digital signal with a reference signal, comprising: a clock input terminal to which an external clock signal is applied; a control signal generating circuit connected to said clock input terminal for generating a control signal from said external clock signal; an integrating circuit connected to receive said inputted digital signal and to receive said control signal from said clock signal generating circuit for integrating said inputted digital signal for a time period defined by said control signal; and a monitor terminal from which to extract a portion of said control signal generated from said control signal generating circuit.
 11. A method of testing an input interface circuit including an input transistor having a control electrode and a pair of current conducting electrodes through which electric current flows when the input transistor is conductive, wherein an input signal to the input interface circuit is to be applied to said control electrode of the input transistor, an integrating capacitor connected in series to said pair of said current conducting electrodes of said input transistor, said input signal being integrated by said input integrating capacitor, and a test transistor having a control electrode and a pair of current conducting electrodes through which electric current flows when the test transistor is conductive, one of said pair of current conducting electrodes is connected to a junction between said input transistor and said integrating capacitor, the method comprising the steps of: operating one of said input and test transistors so that fixed current flows through the pair of current conducting electrodes of said one transistor; operating the other transistor by applying thereto a sequentially varying signal so that current flowing through the pair of current conducting electrodes of said other transistor is sequentially varied; comparing an output of said integrating capacitor with a reference value to determine a logical level of said input signal; and finding a value of said sequentially varying signal when said determined logical level is changed from one state to another.
 12. A method according to claim 11, wherein said one transistor is the input transistor and said sequentially varying signal is applied to the control electrode of said test transistor.
 13. A method according to claim 11, wherein a first constant voltage is applied as said input signal to the control electrode of said input transistor, a second constant voltage is applied to the control electrode of said test transistor, and said sequentially varying signal is applied to the other of said pair of current conducting electrodes of said test transistor.
 14. A method according to claim 11, wherein a first constant voltage is applied to the control electrode of said test transistor, a second constant voltage is applied to the control electrode of the other of said pair of current conducting electrodes of said test transistor, and said sequentially varying signal is applied as said input signal to the control electrode of said input transistor. 